YouTube user brusspup is fascinated by "the science of optical illusions." In his latest video, viewed more than 3 million times over the weekend, he uses shade and light to create an optical illusion on a checkerboard pattern of 25 squares.

The mind-blowing reality that the female model in the video reveals? The light squares and the dark squares are the exact same shade.

CBS shares the science behind his magic trick: "The illusion works because of the 'shadow' that falls on the checkerboard. Lets think of the square that the shadow falls on. Now picture that the shadow doesn't exist, you would see a light colored square surrounded by the dark colored squares. But when the shadow falls on top of that light colored square, the shadow causes the shade to become darker but it also causes the surrounding darker squares to become darker. So within the shadow region the brightness and darkness of the squares are still relative to one another. But they are no longer relative to the squares that lie outside of the shadow."

"What happens in our brain when we view an optical illusion?" brusspup asks. "The information gathered by the eye is processed in the brain to create an idea or image that does not match with a physical measurement of the stimulus source."

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